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Solitary plasmacytoma is characterized by the presence of a single mass of clonal bone marrow plasma cells with no or minimal (<10%) bone marrow plasmacytosis, and no other symptoms except those relating to the primary lesion. Plasmacytomas can also occur as a feature of multiple myeloma (MM) when a lesion occurs alongside one or more myeloma defining events, such as the evidence of end organ damage or a biomarker of malignancy (Section 25.2.1). If a patient with solitary plasmacytoma is found to have at least one myeloma defining event, they would be re-classified as multiple myeloma [42].

About two thirds of solitary plasmacytomas occur in the bone as a single mass, and are termed ‘solitary bone plasmacytoma (SBP)’. The remaining third are extramedullary plasmacytoma (EMP), where the solitary lesion occurs outside the bone in soft tissues such as the head and neck mucosa [1101].

The estimated incidence rate of plasmacytoma is 0.15 per 100,000 per year [473], which represents 3 - 5% of all plasma cell neoplasms. It is approximately twice as common in men as it is in women, and the median age at diagnosis is 60 years, approximately 10 years younger than that of MM patients [473][469][1102]. The median overall survival for solitary plasmacytoma patients is between 7 and 12 years [474].

When solitary plasmacytoma occurs in the bone, the majority (83%) of tumours locate in the axial skeleton, particularly the vertebrae [469], but tumours can also occur in the femur, tibia, pelvis, humerus, skull, ribs and sternum [1103][1104]. An example of a tumour affecting the mandible is presented in Figure 21.1. When solitary plasmacytoma arises outside the bone marrow as an EMP, the tumour is most frequently detected in the head and neck mucosa (85%), but can also occur in the gastrointestinal tract, lung, thyroid, testis, parotid, lymph nodes and central nervous system [1101][1105][1106]. EMP located outside of the head and neck have been linked to an increased risk of dissemination and worse outcome [1101].

Regardless of whether the solitary lesion is located within the bone or soft tissue, the current International Myeloma Working Group (IMWG) guidelines divide solitary plasmacytoma into two categories based on the absence or presence of minimal (<10%) clonal bone marrow plasma cells, as determined by bone marrow aspirate [42].

Local tumour irradiation with or without surgery is the treatment of choice for solitary plasmacytoma, with nearly all patients successfully achieving local control [472]. However, in some patients there is recurrence at the site of the original lesion [471], and around half progress to MM [474][472][471][1107]. Those who do not progress can remain clinically stable for more than a decade [474].

Solitary plasmacytoma patients without bone marrow plasmacytosis have the lowest risk of progression to MM, with 10% progressing 10 years after diagnosis. For those with bone marrow plasmacytosis, the risk of progression increases to 20% for EMP patients and 60% for SBP patients at 10 years [1108][1109]. If all patients with and without plasmacytosis are included, the median time to MM progression for EMP patients is 1.5-2.5 years, compared to 2-3 years for SBP [474].

Up to 5% of patients with solitary plasmacytoma go on to develop multiple lesions in the bone or elsewhere, without evidence of MM [1107][470]. This is referred to as multiple solitary plasmacytoma or macrofocal plasmacytoma [1104].